Stellar Birth and Life Help (page 2)
Introduction to Stellar Birth and Life
All stars evolve from clouds of gas and dust. If the original material in the Universe were perfectly homogeneous—equally dense at every point in space—stars could, in theory, never form. The slightest irregularity, however, brought about more irregularity, leading to regions where the gas and dust became concentrated. This is known as the butterfly effect and was discussed in Chapter 6 when we took a “mind journey” to the planet Mars. This effect breeds new stars.
A Star Is Born
Where the clouds of matter were the most dense, the gravitational attraction among the atoms was the greatest. This caused the dense regions to become even more dense and the sparse regions to get more sparse. A vicious circle ensued, which was repeated in countless locations. It is evidently still taking place in the spiral arms of the Milky Way and in other galaxies.
As a cloud of gas and dust contracts, it eventually begins to heat up. The atoms, originally free to move without restriction, get cramped for space and start to collide with one another. This produces outward pressure, but the increased concentration of matter causes a dramatic increase in the gravitational attraction among the atoms. This gravitational force keeps pulling the gas-and-dust cloud tighter and tighter, and it gets hotter and hotter. Finally, the temperature gets so high that hydrogen atoms begin to fuse, forming helium atoms along with great quantities of energy. This causes the star to become extremely hot, and the outward pressure finally rises to meet the inward force of gravitational collapse. Several hundred thousand years, or a few million years, go by between the initial contraction of the gas-and-dust cloud and the start of the hydrogen fusion reaction.
Large stars are born more quickly than small ones. Sometimes two or more stars are formed so close together that they orbit one another; these are binary stars and multiple stars . Sometimes huge gas-and-dust clouds give rise to clusters of stars.
We can follow the metamorphosis of a young star in an H-R diagram. A protostar , as it contracts and heats up, is not very luminous until the fusion reaction starts. Protostars are situated off the scale at the bottom of the graph. As the protostar contracts to the point where fusion begins, the star’s position moves upward (Fig. 13-4). Almost every new star comes to rest on the main sequence. The most massive stars end up at the upper left and become blue and white supergiants. The least massive stars reach their stable positions at the lower right and become orange and red dwarfs. Oddly enough, the dim, cool, and least spectacular stars burn longest, and the bright, massive, and hot stars have much shorter lifespans.
Once the fusion process begins and the star shines brightly, the remaining gas in the star’s vicinity becomes ionized by ultraviolet (UV) rays and x-rays. When we look at the Pleiades, for example, through a large telescope, this glowing gas can be seen. Much of the superfluous gas and dust near the star is blown away by the stellar wind , high-speed subatomic particles emitted by the star shortly after it begins to shine. This keeps all the gas and dust in the galaxy in a constant state of turmoil, like a room full of smoke in which people are talking and gesturing.
Massive stars lead much different lives than small ones. Our Sun seems to be slightly “on the good side” of the dividing line between stable red and orange dwarfs and unstable blue and white giants.
Eventually, most of the hydrogen in the center of a star gets converted to helium by the fusion process. For nuclear reactions to continue, the core temperature must rise. As the star “runs out of gas,” it starts to cool, and gravitation overcomes the outward pressure caused by internal heating. This contraction causes the temperature to rise again, and this time it goes even higher than it did when the star was born. Eventually, it gets so hot that helium atoms begin to fuse, forming carbon atoms plus energy. Our Sun has not yet reached this stage.
Once the helium has been used up, the star contracts again, becoming hotter still; carbon atoms begin to fuse, forming heavier atoms. In this way, it is believed, all the 92 elements up to and including uranium were formed in the interiors of stars long ago. This includes all those atoms in our planet: all the iron ore, all the silicon, all the gold, and everything else. It even includes the atoms in your body, with the exception of hydrogen that forms part of the water that keeps you alive. But how did all these heavier elements get here from the centers of stars? The answer lies in the fate of the most massive stars: They are doomed to blow up.
Large stars have attracted much attention from astronomers. When stars age, they sometimes swell to extreme size, and their surface temperatures cool. They become red giants. This is expected to happen to our Sun in a few billion years. After the red-giant phase, the Sun will contract and gradually fade away. However, more massive stars undergo a sudden and violent death. For a few days they can become as bright as all the rest of the stars in the galaxy combined. Once in a while we see such a supernova from Earth. When a big star explodes, it hurls much of its matter into interstellar space, where it cools and becomes the stuff from which future star systems can form. Some of these systems are believed to develop into star-and-planet families similar to our Solar System. This explains how the heavy elements got here. If this theory is correct, we all owe our existence to one or more supernovae that took place billions of years ago.
Practice problems of this concept can be found at: Stars and Nebulae Practice Problems
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